Apparatus for controlling voltage sequencing for a power supply having multiple switching regulators

ABSTRACT

An apparatus for controlling voltage sequencing for a power supply having multiple switching regulators includes a first switching regulator, a second switching regulator, a first and second resistors, and a first and second transistors. The first switching regulator provides a positive output voltage at a positive voltage output. The second switching regulator provides a negative output voltage at a negative voltage output. When the voltage at a node between the first and second resistors exceeds a predetermined positive voltage, the first transistor turns on to limit the positive output voltage at the positive voltage output. When the voltage at the node between the first and second resistors drops below a predetermined negative voltage, the second transistor turns on to limit the negative output voltage at the negative voltage output.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to power supplies in general, and inparticular to power supplies having multiple switching regulators. Stillmore particularly, the present invention relates to an apparatus forcontrolling voltage sequencing for a power supply having multipleswitching regulators.

2. Description of the Related Art

In most electronic designs, a circuit board is populated with a numberof electronic circuit devices, each of which may be powered by a singlevoltage source, such as a power supply having a positive voltagepotential referenced to a ground voltage potential. Generally speaking,the electronic circuit devices receive power from the voltage sourcethat is turned on and off in accordance with the requisite usage for theelectronic circuit devices. In practice, it is often necessary tocondition the input voltage from the power supply to be received at theelectronic circuit devices in order to ensure the proper functioning ofthe electronic circuit devices.

Electronic circuit devices having multiple supply voltages are typicallysensitive to voltage sequencing. Hence, problems associated with atypical electronic design usually arise when there is a need to sequencethe electrical power supplied to a number of electronic circuit devicesduring power transitions. For example, the power supply AD588,manufactured by Analog Device, Inc., is a highly accurate voltagereference that requires +15 volt and −5 volt to generate a 10 voltreference voltage for precision circuitry. If during a power-out (ie.,+V1 and −V2 are energized), the +15 volt is up before the −5 volt, the+10 volt reference may move to +13 volt. Likewise, if the −5 volt comesup before the +15 volt, the +10 volt reference may move to −2 volt.

Depending on the application and the circuit configuration, the shiftingof reference voltage can cause very undesirable results such as loss ofcontrol, latch-up, etc. Consequently, it is desirable to provide anapparatus for controlling voltage sequencing for a power supply havingmultiple switching regulators such that reference voltage shifting canbe avoided.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, anapparatus for controlling voltage sequencing for a power supply havingmultiple switching regulators includes a first switching regulator, asecond switching regulator, a first and second resistors, and a firstand second transistors. The first switching regulator provides apositive output voltage at a positive voltage output. The secondswitching regulator provides a negative output voltage at a negativevoltage output. The first resistor and the second resistor are connectedin series between the positive and negative voltage outputs. The firsttransistor and the second transistor are also connected in seriesbetween the positive and negative voltage outputs. The gates of thefirst and second transistors are connected to a node between the firstand second resistors. When the voltage at the node between the first andsecond resistors exceeds a predetermined positive voltage, the firsttransistor turns on to limit the positive output voltage at the positivevoltage output. On the other hand, when the voltage at the node betweenthe first and second resistors drops below a predetermined negativevoltage, the second transistor turns on to limit the negative outputvoltage at the negative voltage output.

All objects, features, and advantages of the present invention willbecome apparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, furtherobjects, and advantages thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment whenread in conjunction with the accompanying drawing, wherein:

FIG. 1 is a circuit diagram of an apparatus for controlling voltagesequencing for a power supply having multiple switching regulators, inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to FIG. 1, there is illustrated a circuit diagram of anapparatus for controlling voltage sequencing for a power supply havingmultiple switching regulators, in accordance with the preferredembodiment of the present invention. As shown, a linear regulatortracking circuit 10 includes a first power supply 11, a second powersupply 12, an NPN transistor Q1, a PNP transistor Q2, a resistor R1 anda resistor R2. Power supplies 11 and 12 are conventional standardpositive and negative linear switching regulators, respectively, as theyare known to those skilled in the art. For the present implementation,first power supply 11 and second power supply 12 are LM317 and LM337,respectively, both manufactured by Fairchild Semiconductor Corporation.A positive input voltage +V1 is input to first power supply 11, and inturn, first power supply 11 generates a regulated positive outputvoltage at a positive voltage output 14. Similarly, a negative voltage−V2 is input to second power supply 12, and in turn, second power supply12 generates a regulated negative output voltage at a negative voltageoutput 15. For the present implementation, under normal conditions, theregulated positive output voltage at positive voltage output 14 ispreferred to be +15 volt, and the regulated negative output voltage atnegative voltage output 15 is preferred to be −5 volt.

Resistors R1 and R2 are divider resistors for determining whentransistors Q1 and Q2 are being turned on. Preferably, transistors Q1and Q2 are PN2222 and PN2907, respectively, both manufactured byFairchild Semiconductor Corporation. Resistors R1 and R2 are connectedin series between positive voltage output 14 and negative voltage output15. The gate of transistor Q1 is connected to a node Vx located betweenresistor R1 and resistor R2. The collector of transistor Q1 is connectedto a voltage control input 16 of first power supply 11. The collector oftransistor Q1 is also connected to positive voltage output 14 via aresistor 21. The emitter of transistor Q1 is connected to ground. Theemitter of transistor Q1 is also connected to voltage control input 16of first power supply 11 via a resistor 22. In addition, the emitter oftransistor Q1 is connected to positive voltage output 14 via resistor 21and resistor 22.

The gate of transistor Q2 is connected to node Vx. The collector oftransistor Q2 is connected to a voltage control input 17 of second powersupply 12. The collector of transistor Q2 is also connected to negativevoltage output 15 via a resistor 24. The emitter of transistor Q2 isconnected to ground. The emitter of transistor Q2 is also connected tovoltage control input 17 of second power supply 12 via a resistor 23. Inaddition, the emitter of transistor Q2 is connected to negative voltageoutput 15 via resistor 23 and resistor 24.

For the present implementation, the value of resistor R1 is preferably15 KΩ and the value of resistor R2 is preferably 5 KΩ. Also, the valueof resistor 21 is preferably 240 Ω, the value of resistor 22 ispreferably 2.61 K Ω, the value of resistor 23 is preferably 750 Ω andthe value of resistor 24 is preferably 240 Ω.

When first power supply 11 and second power supply 12 are operatingunder normal operating conditions, node Vx is at 0.0 volt and bothtransistors Q1, Q2 are turned off. If the positive output voltage atpositive voltage output 14 starts to come up while negative outputvoltage at negative voltage output 15 is still down, then the voltage atnode Vx will increase to approximately 0.7 volt and transistor Q1 willbe turned on. Under normal operating conditions, the voltage acrossresistor 21 is approximately +1.2 V (for voltage control input 16) tomaintain normal voltage output from first power supply 11 to positivevoltage output 14. However, when transistor Q1 is turned on, the voltageacross resistor 21 increases to approximately +15 V. As a result, firstpower supply 11 limits its +15 volt positive output voltage at positivevoltage output 14.

On the other hand, if the negative output voltage at negative voltageoutput 15 starts to come up while the positive output voltage atpositive voltage output 14 is still down, then the voltage on node Vxwill drop to approximately −0.7 volt, and transistor Q2 will be turnedon. Under normal operating conditions, the voltage across resistor 24 isapproximately −1.2 V (for voltage control input 16) to maintain normalvoltage output from second power supply 12 to negative voltage output15. However, when transistor Q2 is turned on, the voltage acrossresistor 24 decreases to approximately −5 V. As a result, 6 second powersupply 12 limits its −5 volt negative output voltage at negative voltageoutput 15. For example, when the negative output voltage at negativevoltage output 15 reaches −4 volt, the positive output voltage atpositive voltage output 14 should be between +10 volt and +15 volt.

As has been described, the present invention provides an apparatus forcontrolling voltage sequencing for multiple switching regulators.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

1. An apparatus for controlling voltage sequencing for a power supplyhaving multiple switching regulators, said apparatus comprising: a firstregulator for providing a positive output voltage at a positive voltageoutput; a second regulator for providing a negative output voltage at anegative voltage output; a first resistor and a second resistorconnected in series between said positive and negative voltage outputs;and a first transistor and a second transistor connected in seriesbetween said positive and negative voltage outputs, wherein gates ofsaid first and second transistors are connected to a node between saidfirst and second resistors, wherein said first transistor turns on tolimit said positive output voltage at said positive voltage output inresponse to a voltage at said node between said first and secondresistors exceeds a predetermined positive voltage, wherein said secondtransistor turns on to limit said negative output voltage at saidnegative voltage output in response to a voltage at said node betweensaid first and second resistors drops below a predetermined negativevoltage.
 2. The apparatus of claim 1, wherein said first transistor isan NPN transistor.
 3. The apparatus of claim 1, wherein said secondtransistor is a PNP transistor.
 4. The apparatus of claim 1, whereinsaid first and second regulators are switching regulators.
 5. Theapparatus of claim 1, wherein said positive output voltage isapproximately +15 volt and said negative output voltage is approximately−5 volt.
 6. The apparatus of claim 4, wherein said predeterminedpositive voltage is approximately +0.7 volt and said predeterminednegative voltage is approximately −0.7 volt.
 7. The apparatus of claim1, wherein said first resistor is 15 KΩ and said second resistor is 5KΩ.
 8. The apparatus of claim 1, wherein said apparatus further includesa first set of biasing resistors connected between said first transistorand said positive voltage output.
 9. The apparatus of claim 8, whereinsaid apparatus further includes a second set of biasing resistorsconnected between said second transistor and said negative voltageoutput.